1. Field of the Invention
The present invention relates to the field of optical fibers for the generation and/or amplification and transmission of high power laser energy.
2. Information Disclosure Statement
Many applications require or benefit from the use of a high-power pump-light source. Use of a conventional laser-active single mode fiber is insufficient to launch high-power pump radiation in order to achieve high-power laser emission. There are numerous techniques available for increasing a fiber's output power, such as attempts to reduce energy density at fiber end faces through devices such as tapered fiber tips. Another common technique to overcome this obstacle is the use of appropriate so-called double-clad fibers. The use of a double-clad fiber allows coupling of pump radiation with the inner clad, which is often referred to as the pump core. The pump radiation is contained within by a polymer coating that acts both as a protective coating and as an outer clad. The pump light is coupled to the inner clad where it propagates within-the inner core and passes through the fiber core. Dopant ions (preferably rare earth (RE) ions) within the active core (i. e. the fiber core) absorb energy (i. e. pump radiation either by direct absorption or by energy transfer or by upconversion or by similar processes), which is released when pump radiation is transmitted within the core. This energy is converted to new (laser) photons that act to strengthen the laser beam and to increase its power output. The use of a higher-diameter multi mode inner clad allows for coupling with higher-power and/or lower-beam-quality pump radiation, as usually provided by high-power laser diode systems.
Unfortunately, higher order modes as they appear at large pump core diameters carry only little or even no energy at the center of the pump core where the active core is usually placed. Therefore, measures, such as an off-center placement of the active core, a non-circular pump core etc., have to be taken to improve the absorption of pump radiation within the active area.
A technique to increase the pump power that can be launched into the pump core is the so-called polarization coupling technique. Usually two pump light sources with perpendicular oriented polarization directions are coupled to the pump core. In general, this doubles the available pump power.
Further, wavelength division multiplexing is an option for improving the transmission of fibers. Two or more pump light sources with (slightly) differing wavelengths are coupled to the pump core, e. g. by dichroic beam combiners. Multiplexing allows the pump power to be increased up to a few hundred watts. However, multiplexing alone is not sufficient to increase the transmission capacity of the fiber, in that the power density in the active core can exceed about 100 MW/cm2 which causes thermal loading of the fiber itself due to residual absorption of the evanescent field of the pump radiation within the polymer coating.
The current approach in the art to increase the output power of optical fibers using double-clad structures is illustrated in FIG. 1. In this illustration, active core 104 that has a high refractive index is surrounded by the inner clad 106 that acts as pump core. This pump core has a lower refractive index that allows for waveguiding of the laser radiation within the core 104 which is emitted by the dopant ions within said core 104. Active core 104 is made, for example, from rare earth-doped (“RE-doped”) SiO2, and pump core 106 is made, for example, from pure silica. Usually, the active core 104 is a single mode structure, whereas the pump core 106 is a (highly) multi-modal structure that is designed to match the beam characteristics (e. g. diameter and divergence) of the used pump laser source. The active core 104 is generally of a circular shape. It can be placed either in the center of the pump core 106 or in an off-center position which might be useful to increase the absorption efficiency. Pump core 106 is of a circular or any other cross section (e. g. rectangular, D-shaped, etc.) that might be advantageous to improve the absorption efficiency. The two-core structure is surrounded by a (protective) coating layer 102 made from a polymer such as silicone. As the refractive index difference between coating 102 and pump core 106 is responsible for guiding the pump radiation, coating 102 functions as a cladding as well as a protective layer. Therefore, the evanescent field of the pump radiation leaks into the coating 102. This is a problem because polymers' damage threshold is usually significantly lower than the damage threshold of silica. This often results in the destruction of the optically pumped double-clad fiber, especially of its end faces.
U.S. Pat. No. 6,157,763, describes a double-clad fiber designed to maximize the pumping ability of the fiber by shaping the pump cladding in a specific way and introducing stresses during the manufacturing phase to stimulate mode mixing and maximize absorbed energy in the core. Like other double-clad fibers, the fiber in this patent contains a doped core surrounded by a doped cladding with a lower index. The doped cladding is then surrounded by a cladding layer with a further lower index material. This invention envisions the use of polymers as the material of the outer cladding, which makes this invention susceptible to the kind of degradation discussed above.
U.S. Pat. No. 5,291,501 is another example of a double-clad fiber generally described in FIG. 1. This patent describes a fiber with an active core surrounded by an active inner cladding and a second outer cladding. Radiation coupled to the inner cladding causes laser emission which leaks into the monomode core, where it serves as the pump radiation for the laser active dopant in the active core.
As is described in World Patent Application WO 99/30391, double-clad fibers suffer from various problems. Double-clad fibers with concentric, radially symmetric layers have proven to be an inefficient way to amplify signals. The result of this was the creation of circular waveguides with off-center cores or non-circular pump cores. The creation of these shaped fibers resulted generally in waveguides with polymer outer claddings. Polymer claddings have a significantly lower damage threshold than silica and can be easily damaged by pump radiation.
WO 99/30391 purports to solve these problems by providing a double-clad optical fiber consisting of a glass core, an inner glass cladding layer containing an irregular outer boundary, and an outer glass cladding layer whose inner boundary conforms to the shape of the inner layer's outer boundary. A third cladding layer is optionally provided between the core and inner cladding layer to suppress high order core modes.
The outer glass cladding layer acts as both a cladding to contain the pumping radiation and an outer covering of the fiber.
U.S. Pat. No. 5,966,491 describes an optical fiber with a circular pump core, a pump cladding characterized by longitudinal troughs, a second cladding surrounding the pump cladding, and a third cladding surrounding the second cladding. In one example, the core, pump cladding, and second cladding are silica glass, and the third cladding is made of a polymer. This invention appears to be concerned with directing energy into the pump cladding and especially into the active core, rather than protecting the polymeric third cladding.
U.S. Pat. Nos. 5,949,941 and 5,966,491 describe a double-clad laser fiber comprising a rare earth-doped core, a non-circular inner cladding, a circular outer cladding, and an outer polymer layer. The outer cladding layer has an index of refraction that is lower than the inner cladding and preferably as low as possible. Generally, in the prior art according to this patent, the outer layer is used as a cladding to reflect light back into the core.
There is thus a need for multi-clad optical fibers capable of transmitting high-power laser emissions without the need for sophisticated waveguide profile shaping and without the need for claddings that are highly stable or high-strength with respect to the guided optical power. The present invention satisfies this need.